This invention relates to electrically heating a body of molten material by means of induction. The invention is applicable to the melting of a material which is susceptible to having electric currents induced therein, and it is particularly applicable to the melting of glass or the like.
It is well known that material may be heated by induced currents when the material is placed within a coil carrying alternating current. An advantage of this type of heating is that the material being heated does not come into contact with the electrical source; e.g., electrodes need not be immersed in the melt. The general concept of inductively heating glass has been disclosed in many patents, for example, U.S. Pat. Nos. 1,830,481; 1,906,594; 3,205,292; and 3,244,495. Much of the prior art is limited to small scale embodiments, and large-scale melting of glass by induction heating has not found significant commercial acceptance. Thermal energy from combustion of fuel has generally been more economical than electric energy for glass melting. Moreover, induction heating has sometimes been considered to entail low efficiency in transferring electrical power into thermal energy. Also, it has been the belief of some in the art that large scale induction heating of glass would entail a prohibitively large induction coil.
In U.S. Pat. No. 4,610,711 (Matesa et al.) there is disclosed a more economical approach to using induction heating in a glassmaking process, wherein the induction heating is limited to raising the temperature of glass to its peak refining temperature at a downstream portion of the process. It would be desirable to utilize induction heating effectively at other stages of the glassmaking process.
Induction heating is capable of generating very high power densities, which can yield very high rates of energy transfer. Induction melting requires a vessel to hold the melt, and this vessel is also heated by the electric field. At high rates of heating, cooling of the vessel may be required to preserve its integrity or to retard contamination of the melt by erosion of the vessel. The cooling, however, reduces the efficiency of the heating process. An arrangement that maximizes melt purity at the expense of efficiency is shown in U.S. Pat. No. 3,461,215 (Reboux). There, cooling elements comprise the vessel and are in direct contact with the material being melted, whereby the rate of heat extraction is sufficient to maintain a layer of the material unmelted. The avoidance of contamination makes such an arrangement attractive for production of high quality glass, but for commercial mass production, the large energy losses would be economically prohibitive.